In a semiconductor device of a lead frame type such as QFP (quad flat package), the bottom face of a semiconductor element is die-bonded to a die pad of a metal lead frame, bonding pads on an upper surface of the semiconductor element are wire-bonded to the leads of the lead frame, and the semiconductor element and inner portions of the leads are molded in a plastic mold. The semiconductor element is directly mounted on the die pad of the lead frame. The die is continuous with two or four lead-like structures (called external leads), and the external leads extend to the outer surface of the plastic mold (interface between the plastic mold and the external atmosphere). Most of heat generated in the semiconductor element is dissipated via these external leads. Thus, there is less necessity of providing heat dissipating member such as a metal plate above the upper surface of the semiconductor element.
In a semiconductor device of a QFP type, leads project from side walls of the package. Therefore, as the number of leads increases, the size of a package becomes large. When the size is to be reduced, lead pitch will become narrow, posing a problem such as difficulty of mounting the package to a printed circuit board (PCB). It is difficult to manufacture a package having 500 pins or more.
There is a package of the type called ball grid array (BGA), which allows an increase in the number of leads. In the ball grid array, a semiconductor element is mounted on a substrate having electrodes and wirings, and the bonding pads of the semiconductor element are wire-bonded to the electrodes of the substrate. Solder balls are disposed on electrodes on the bottom surface of the substrate, for example in lattice pattern. Since solder balls are disposed on a flat plane in array shape, it is easy to deal with an increase in the number of pins. However, bonding pads are formed in the peripheral area of a semiconductor chip, and wire-bonded to electrodes of the substrate. There is therefore a limit in increase of the number of pins. This package structure is suitable for the number of pins from about 200 to about 1000.
The structure capable of dealing with an increased number of pins is flip-chip bonding wherein electrodes are disposed in an array shape on the upper surface of a semiconductor element, and the semiconductor element turned upside down is connected to the electrodes of the substrate via solder balls. In this case, it is difficult to provide general versatility to the substrate. It is possible for the ball grid array to use a substrate of general versatility, and the ball grid array has an intermediate structure between the lead frame and flip-chip bonding from the view point of the number of pins.
A BGA structure having resin mold body is called a plastic ball grid array (PBGA). Plastic mold of epoxy resin or the like seals the substrate surface, covers the semiconductor element and bonding wires. The substrate has a structure that electrodes and wirings are disposed on insulating member of such as glass epoxy. Since the semiconductor element will be enclosed with resin material, thermal conductivity will be low as a whole. If a heat generation amount in the semiconductor element is large, some structure is desired to positively dissipate heat generated on the surface side of a semiconductor chip.
Japanese Patent publication No. 8-139223 describes a BGA semiconductor device in which ball electrodes are disposed on the bottom surface of the substrate in a matrix shape, a semiconductor element is mounted on the front surface of the substrate, and resin body seals the semiconductor element and the bonding wires, which further includes a heat conductor (cap) buried in the sealing resin. For example, a cap made of good heat conductivity such as Cu, Cu alloy, Al, Al alloy and Fe—Ni alloy is mounted on the substrate surface on which the semiconductor element is mounted, covering the whole upper surface and side walls of the semiconductor element. A number of openings having desired shapes such as circle are formed in the upper plane and side walls of the metal cap to facilitate injection of sealing resin into a space between the cap and semiconductor element. It also discloses a structure that the cap upper plane is exposed to the resin surface, and a structure that the cap is lowered nearer to the semiconductor element in a region inside the bonding pads of the semiconductor element.
U.S. Pat. No. 7,126,218 discloses a heat spreader brought in contact with an inner area of a semiconductor element inside the areas of the bonding pads, in which the heat spreader is raised in peripheral areas and outside thereof of the semiconductor element to avoid contact with the bonding wires. It discloses a structure that the outer periphery of the heat spreader is connected to the substrate and another structure that the outer periphery of the heat spreader is floated in sealing resin body.
U.S. Pat. No. 7,432,586 describes a structure in which a semiconductor element is mounted on a stiffener, a cap is mounted on the stiffener, covering the semiconductor element, the stiffener is mounted on a substrate, and the semiconductor element is sealed in resin. The stiffener and cap are made of metal, alloy or the like having thermal conductivity and electric conductivity or ceramics, metallized plastic or the like. The stiffener and cap constitute an enclosure of the semiconductor element, dissipating heat generated by IC operation and preventing electromagnetic interference.
Thermal conductors disposed above the semiconductor elements are called cap, heat spreader, heat sink or the like. In this specification, the thermal conductor is most often called heat spreader, putting heat dissipation function as main character.